Method of sealing magnesium cells

ABSTRACT

In sealing a dry cell having an anode in the form of a cylindrical magnesium container and a sealing disk of a plastic material having its periphery in sealing engagement with the open top portion of the magnesium container, a restricted aperture is provided in the sealing disk of a size sufficient to permit gases formed in the cell immediately after assembly to escape, yet sufficiently small to prevent the venting of excessive moisture. The aperture is closed to seal the cell after the initial gas formation period by reducing the radius of the open top of the cell and the disk therein sufficiently to close the aperture and seal the cell.

United States Patent [72] Inventors Milton E. Wilke Freeport, 111.; Howard J. Strauss, Beachwood, Ohio [21] Appl. No. 13,301 [22] Filed Feb. 24, 1970 [45] Patented Oct. 26, 1971 [73] Assignee Clevite Corporation Cleveland, Ohio Continuation-impart 01 application Ser. No. 704,001, Feb. 8, 1968, now abandoned.

[54] METHOD OF SEALING MAGNESIUM CELLS 1 Claim, 4 Drawing Figs.

[52] U.S. Cl 136/175, 136/100 M [51] 1nt.Cl 1101m 1/02 [50] Field of Search 136/100,

Primary Examiner-Donald L. Walton Attorney-Darbo, Robertson & Vandenburgh ABSTRACT: In sealing a dry cell having an anode in the form of a cylindrical magnesium container and a sealing disk of a plastic material having its periphery in sealing engagement with the open top portion of the magnesium container, a restricted aperture is provided in the sealing disk of a size sufficient to permit gases formed in the cell immediately after assembly to escape, yet sufiiciently small to prevent the venting of excessive moisture. The aperture is closed to seal the cell after the initial gas formation period by reducing the radius of the open top of the cell and the disk therein sufficiently to close the aperture and seal the cell.

RELATED APPLICATION This application is a continuation-in-part of application, Ser. No. 704,001, filed Feb. 8, 1968, now abandoned.

BACKGROUND AND SUMMARY .OF THE INVENTION This invention relates to improvements in dry cells wherein magnesium is used as the anode metal, the invention being directed particularly to an improved seal arrangement which permits gas venting during the initial .period after assembly, and which pennitsa hermetical seal to be formed thereafter to prevent leakage during use of the cell.

While magnesium has long been recognized as a metal having many advantages when employed as the negative electrode for the electrochemical system of dry batteries, certain physical and chemical peculiarities of this metal make it more difficult to use than zinc. One of the most difficult problems encountered in the use of magnesium is its extremely great reactivity, particularly when exposed to aqueous solutions. It has been found that after the assembly and sealing of a magnesium cell, and during early storage, gases are produced, principally hydrogen, which must be .vented in order to avoid the buildup of excessive internal pressure. The formation of hydrogen ceases after a short period of time. Consequently, at the end of this period the cell can be hermetically sealed.

In order to permit the venting of gases during the initial gas formation, attempts have been made in the prior art to permit the cell to stand without the sealing closure, and then to seal the cell when gas formation has stopped. However, it has been found that there is an excessive moisture loss during the period when the cell stands in the unsealed condition. In an attempt to prevent moisture loss while still permitting the venting of gases, it has been attempted to regulate the degree of closure between the top of the anode can and the plastic disk to which it is eventually sealed so that gases may escape during the gas formation period while still retaining moisture within the cell. However, it has been found that, although this expedient is feasible in the laboratory, in practical production the tolerances required of each of the mating components are such that the closure is frequently rendered either too tight to permit gas leakage or so loose that it permits excessive ventilation with loss of moisture and may also result in mechanical problems due to relative shifting of position of the parts.

It is an object of the invention to provide a method for making a magnesium cell whereby gases formed during the initial period after assembly are permitted to escape in order to prevent the buildup of excessive gas pressure after which the cell is properly sealed so that leakage will not subsequently take place.

It is a further object to provide a magnesium cell closure structure which can accomplish the desired ends and which still can be constructed and subsequently properly sealed within the limitations of mass production.

Other objects and advantages of the invention will become apparent from the following description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a cross-sectional view of a cell according to one embodiment of the invention;

FIG. 2 is a plan view of a closure disk used in the cell of FIG. 1;

FIG. 3 is a cross-sectional view of the cell of FIG. I after final sealing, and

FIG. 4 is a plan view of an alternative form of closure disk.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. I and 2 of the drawing, one embodiment of the cell of the present invention is shown comprising a tubular magnesium can having a sealing disk 2 formed of a suitable stiff but somewhat resilient plastic composition inserted in clo- 7 sure position in theopen top of the can. The sealing disk 2 is provided with a hole 3 having a carbon rod 4 disposed therethrough. The carbon rod is provided at its end with a metal cap 5 of a material such as brass. The sealing disk 2 is additionally provided with a restricted aperture in the form of a notch 6 extending across the thickness of the disk. The aperture is of sufficient size to ensure a minimum passageway for a gas such as hydrogen to pass theret'hrough when the disk is placed in the top of can 1, but sufficiently small to prevent a substantial amount of water vapor from escaping. The cell additionally contains a paper separator 7 having a suitable cell electrolyte absorbed therein. Disposed within the paper separator is a depolarizer core 8 of a composition comprised of about nine parts of a depolarizer material such as manganese dioxide and one part of an acetylene black, and additionally including a suitable electrolyte such as magnesium :chloride. A paper washer 9 covers one end of the depolarizer core.

The cell is assembled by inserting the separator 7 in the open can 1. The depolarizer cathode 8 is then inserted inside the separator and suitably tamped. The washer 9 may then be placed over the cathode 8. The brass-capped carbon rod 4 having the sealing disk 2 arranged thereon is inserted through a hole provided in the washer 9 and into the cathode material.

After the cell has been thus assembled, it is permitted to stand for a suitable length of time, as for example 4 or 5 days, to permit gases which initially form when the various component parts and materials of the cell are first brought together to escape from the cell through the aperture 6 prior to the final sealing. At the end of the period, the cell is hermetically sealed. To this end the top portion 10 of the can is constricted or reduced in diameter (see FIG. 3) by means of a suitable tool which applies radial pressure thereto to make it smaller and compress the disk and place it under radial strain and thus provide sealing pressure at the interfaces between the disk 2 and the carbon rod 4 and top portion 10 of the can, respectively. The composition and thickness of the disk 2 is such that upon removal of the forming tool, the end portion 10 of the can retains its deformed shape with reduced diameter. In this process, the radius of the closure disk is reduced somewhat more than the depth of the notch 6 so that the gas escape aperture which it provided is completely closed. To complete the closure and ensure a permanent seal, the ring 11 of steel or other suitable material is forced onto the reduced portion 10 of the magnesium can. To facilitate the application of the ring 11 to the end of the can, the end portion 10 of the can and/or the engaging portion of the ring 11 may be slightly tapered. The cell, thus sealed, is ready for service.

An alternative form of closure disk is shown in FIG. 4. In this further example of the invention, the aperture, instead of being a notch in the outer periphery of disk 2, is a notch or scratch provided at the periphery of the hole 3 in which the carbon rod is to be inserted; i.e., a notch in the inner periphery of the disk. The cell is assembled in the same manner as previously described with reference to the embodiment of FIG. I. When, at the end of the gassing period, radial constriction or clamping force is applied to the end of the can and a holding ring 11 applied, as above described, the aperture formed by notch 12 is closed and the disk-carbon rod interface securely sealed under pressure.

The sealing means and method of the present invention in the exemplary embodiments have a number of advantages over prior art methods for accomplishing the same function. The restricted aperture may be provided in suitable manner, as during the extrusion process utilized in forming the sealing disks. The size of the apertures is not extremely critical, the requirement being that they provide with certainty a path for the escape of gases while being otherwise of minimum size to close tightly when the cell is finally squeezed shut.

Althoughthe present invention has been described in only two alternative forms, variations thereof may be practiced those skilled in the art without departing from the spirit or bling the cell including arranging said disk in the top portion of the can, allowing the cell to age and the gases formed during aging to escape through said aperture, and thereafter applying inward radial pressure to the top portion of the cell to reduce the diameter thereof and of said disk sufficiently to close said aperture and permanently seal the cell at the diskrod and disk-can interfaces.

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